Coherent optical receivers are sometimes deployed to detect phase modulated optical signals. In such receivers, a local oscillator (e.g., a laser) may be provided to supply light, which is continuous wave (CW) and has a wavelength which may be the same as or close to the wavelength of the phase modulated optical signals. In the receiver, the local oscillator light and the incoming phase modulated light are supplied to a hybrid mixer, also referred to as a 90 degree optical hybrid, which outputs in-phase and quadrature components of the incoming phase modulated optical signals. The in-phase and quadrature components may have substantially the same optical power upon being output from the hybrid mixer.
The incoming phase modulated optical signals may be polarization multiplexed. A polarization beam splitter may be provided to separate the incoming signals into constituent separate first and second polarization components (e.g., transverse magnetic (TM) and transverse electric (TE) components). The hybrid mixer may receive such TE and TM components as separate inputs. Alternatively, first and second hybrid mixers may be provided that receive the TE and TM components respectively, as well as the local oscillator light.
When implemented with waveguides, such as in a photonic integrated circuit (PIC) or a planar lightwave circuit, the hybrid mixer may include portions in which such waveguides cross one another to maintain a phase difference between the quadrature components to match a phase of the in-phase components. At such crossings, the light in one waveguide may interfere with the light in the other waveguide. In addition, the crossings may introduce loss. Accordingly, the waveguide crossings may cause an undesirable level of signal degradation.